In 2008, the Iowa Department of Natural Resources issued a consent order to the city of Marshalltown requiring elimination of sanitary sewer overflows and basement backups. In response, the city initiated a program to reduce infiltration/inflow (I&I) in the sanitary sewer system and provide collection system improvements for conveyance of up to a 10-year storm event.

In 2011, the city asked FOX Engineering to assist in reviewing the city’s interceptor and pump station capacities. The system evaluation indicated that significant improvements were necessary to convey the 10-year event to the Water Pollution Control Plant (WPCP).

Solutions:

Several projects resulted from this study, including the Turner Street Pump Station Improvements and Force Main Project. The objective was to eliminate bypassing of up to 11 million gallons per day (MGD) of sanitary sewer flows from the Turner Street pump station to Linn Creek during a 10-year storm event. The modified pump station and new force main divert up to 19 MGD of sanitary sewer flows to one of the city’s equalization basins (which have a 43 MGD total capacity) at the WPCP.

The FOX team encountered numerous design challenges with the existing pumping facilities and proposed force main alignment. The pump facility was constructed in 1972 by the Army Corp of Engineers (ACOE) to allow for sanitary sewer flows from the southeastern area of the city to be pumped across the constructed Linn Creek channel and flood protection system. Since this project required pumping capacity above the capability of the existing pumps, new pumps were needed that would provide the necessary capacity under the new head conditions and fit within the limited space available in the pump station building. Complicating design efforts, much of the proposed alignment needed to be installed adjacent to an existing ACOE levee system and a portion of that alignment was on Union Pacific Railroad (UPRR) property. This required lengthy permitting efforts from both the ACOE and from the UPRR.

The design team separated construction into two projects. First, improvements to Turner Street Pump Station included removal of all (3) pumps and installation of four new pumps, new electrical service and motor control center, a new outdoor standby generator, miscellaneous demolition, piping changes, and control system modifications. This phase was bid in June 2013 for a total construction cost of $870,000, and it was substantially complete by December of 2014.

The second project included installation of 9,100 LF of 30-inch sanitary sewer force main, 483 LF of 30-inch sanitary sewer force main with bore and jack steel casing under the UP Railroad and under three city streets, and approximately 1,200 feet of complete street section removal and replacement. To accommodate infrequent use of the force main, cleanout and drain manholes were necessary along the alignment to allow maintenance of the force main system. The project was bid on June 21, 2016; low bid was $4,215,155.

Construction commenced on August 25, 2016 with dewatering well installation along the ACOE levee on E. State Street to the WPCP. Nearly all force main piping, jack and bore road crossings, and 4,300 square yards of mainline concrete pavement were installed on East State Street by December 1st.

Benefits:

The force main project provides peace of mind to a community beset by rain events that were difficult to control by eliminating bypassing to Linn Creek. Adding to the positives on the project, the force main project was substantially complete by February 8, 2017, five months ahead of schedule. The costs to date indicate that the construction cost will likely be less than the contract price. Final completion is planned for July 2017.